The present invention relates to a structure of a superconductive thin film field effect transistor and to a high yield and productive fabrication method of the same.
Conventionally, Nb-Ge metallic material (for example Nb.sub.3 Ge) is used as a superconductive material. Since these materials have relatively large coherent length, a Josephson device in which an insulator is pinched between superconductive materials can be fabricated relatively easily, and such devices as a transistor using proximity effect can be made. Superconductive materials using conventional metals also have an advantage in that, since they have no anisotropy in their structure, electronic devices can be fabricated without considering orientation and other related issues.
However, superconductive materials using these metallic materials have a low Tc (superconductive critical temperature is referred to simply as Tc hereinafter), such as 23 K. or less. In applying them to industrial uses, they would clearly be more useful if their Tc were above the temperature of liquid nitrogen (77 K.). To that end, it has been being strongly desired to build up electronic devices using oxide superconductive materials.
Nevertheless, electronic devices using oxide superconductive materials have not been constructed so far, mainly due to the following two reasons: First, high Tc superconductive materials have problems in that their coherent length is short and they have anisotropy in their crystal structure; and second, high temperature annealing is necessary in processing them. The former problem requires that the devices be processed precisely, and renders processing of the devices difficult. The latter problem causes diffusion of atoms in annealing and thereby adversely affects characteristics of the devices.
More recently, a phenomenon called a long distance proximity effect has been confirmed. When a non-superconductive material having a perovskite structure which is like a parent material of oxide superconductive materials is enclosed between a plurality of oxide superconductive materials, superconductive current flows even if the distance between the oxide superconductive materials is larger than that in the conventional proximity effect. The cause of this phenomenon remains unclear, though by using the long distant proximity effect, the problem of coherent length of the oxide superconductive materials may possibly be overcome, so that such electronic devices as transistors using the oxide superconductive materials may be fabricated. Presently, however, electronic devices using the long distance proximity effect have not been obtained.
Accordingly, it is an object of the present invention to establish a structure of and a process for fabricating such electronic devices, including superconductive transistors in particular, using the aforementioned long distance proximity effect.
It is obvious that it is desirable to use a mono-crystal thin film of an oxide superconductive material whose coherent length is long in the direction of ab plane; thus, a planar type structure using c-axis oriented film is naturally desirable. However, since a process for fabricating such an ideal structure has not been known, only laminated type electronic devices, whose coherent length is short and junctioned in the ab-axis direction, and incomplete planar-type electronic devices have been obtained.
Furthermore, in terms of the process for fabricating them, resists and others used in prior art semiconductor fabricating processes cannot sustain the film forming temperature of the oxide superconductive materials.